Offense:
21

Defense:
0

Utility:
9

Introduction

Hey guys, I go by Bropeth and I'm here to show you how I build and play Kassadin. He's recently become my favorite champion by far, and I build him a little different than most people that I see. So, even though it's my first one, I thought I would write a guide and show Mobafire how I build Kassadin to snowball and nuke like the Lovecraftian badass he is.
There's nothing I can say about Kassadin himself that isn't outlined better in other guides, so I won't bother. This is a build and playstyle guide, and if you don't know anything about Kassadin it isn't the guide for you.

Pros / Cons

Pros:

-Great burst
-High mana pool
-Absurd mobility

Cons:

-Squishy
-Very hard to master

The Proof

Here's a few games I've played in the past couple of days. My build works well for me, and I'm sure it can work just as well for others too.

Masteries

Once again, I go for a pretty standard 9/0/21 build. Deviate from this however you'd like; it's all preference like with the runes. I personally like the spell vamp in the new masteries quite a bit, as it helps Kassadin do better against champions with sustain in mid. The mana and mana regen masteries are also helpful for sustain in the lane, especially since this build lacks the Catalyst the Protector that most people buy.

On Dominion I go for a 21/0/9 build instead, as the Crystal Scar aura grants extra mana regen.

Summoner Spells

Preferred:
Flash is a **** move on Kassadin. Some people discredit it due to the fact that Kassadin already has a blink every few seconds, but I say that only adds to its effectiveness. You can into a fight and right back out if things go awry . Sometimes even the tiny cooldown on Riftwalk is too much to safely escape. Flash is also how I get my crucial early-game kill in most games, which I'll expound upon in the Playstyle section. Overall, this is a fantastic spell even for Kassadin.
Ignite is self-explanatory. It helps Kassadin net those kills he would otherwise have barely missed. When Kassadin has Ignite, no one gets away.

Other Viable Picks:
This spell is actually great on Kassadin, but I prefer buying Quicksilver Sash for the suppression cleanse if I need it.
This is quite helpful for ganking lanes, pushing lanes, getting back to your lane after going to buy items, etc. Overall a great spell, just not one that I find myself using the most.

Back when I was a nooblord Kassadin I ran Clarity every game and was constantly scoffed at for it. I kind of understand why now, but Clarity can actually be a lifesaver in certain situations. It functions as an escape mechanism more than anything else. When you find yourself OOM with a Xin Zhao running at you full-speed, you can pop Clarity and the hell out of there.

I'm personally not a fan of this one on Kassadin, but I've seen some people use it effectively, so I won't knock it. You can use the slow to keep people from getting away, but considering Kassadin's mobility it should mainly be used to stop their AD carry from tearing you to shreds while is on cooldown.

Items

Start out with a Sapphire Crystal and Health Potionx2. The crystal will later build into your first Archangel's Staff, but don't rush that item first. By the first time you go back, you should be able to finish your Tear of the Goddess. Do this as soon as possible but don't finish it into AA for a while. It's more efficient to buy other items first. After your Tear, buy level one boots, and if you're doing decently in mid buy a Mejai's Soulstealer as soon as possible. This is what allows Kassadin to snowball the way he does, as picking up stacks is relatively easy for him. After this, finish the boots into Sorcerer's Shoes, build your Rabadon's Deathcap, and finish off your Archangel's Staff or buy the Void Staff first if they're already building magic resist. Once you have these core items, the game may very well be over. If not, buy another Archangel's Staff and lol at your damage output.

Playstyle

Let's get this out of the way first: My Kassadin build makes him a true glass cannon. He has little survivability outside of his escape mechanism and will die extremely fast if attacked. The reason why is simple: Maximum damage potential. This is achieved through an Archangel's Staff stack, though not quite to the degree you sometimes see on Karthus. I love Archangel's Staff on Kassadin. Some people buy Rod of Ages and CDR on him, but I find it far more entertaining and effective to Riftwalk around the map with a nearly endless mana pool and absurdly high AP. When you can Riftwalk in and nuke half their team before they know what hit them, you don't need a whole lot of health.

Start out the game by calling mid. Kassadin belongs in a solo lane, and it's possible to do well there even against AD ranged champions like Caitlyn if you do it right.
Pick up your first and use it to harrass, then get . Unlike most Kass players, I grab a rank in third for the mana regen component. If at all possible, zone your opponent by autoattacking the creeps and using your Q to harass whenever they come within range. However, if you find yourself being out-harassed, stay further back and use your Q to last-hit creeps. Harass as much as you can without depleting too much mana, and autoattack the creep waves as much as you can to keep your mana up. When the enemy champion is at about 1/4-1/5 health, you go for a kill. This can be done even before level six. First, wait until is at the ready. Then, in and ++. While the slow from is active, hit and autoattack the champion a few times. This should be more than enough to net you an early kill, after which you can finish farming the creep wave and go back to buy items.
If you've managed to effectively harass and your enemy is at low health when you hit level 6, in and +++ like you did with your earlier. After you get your R, start ganking at any possible opportunity. Kassadin's farm is less than spectacular early-mid game, so you need the gold to stay ahead on your build.

If all goes well and you get kills, you should be a few items into your build and doing ******ed amounts of damage. In a teamfight you want to in, silence their caster with , and hit as many as you can with , then walk away. After a few seconds when your cooldowns are gone, go in again and nuke anyone low enough to kill.

Situational Items

Quicksilver Sash
One word: Malzahar. If you find yourself against a Malz or a champion with a similar hard CC that you keep dying to, buy this item. The cleanse is even better than the summoner spell and will allow you to escape some sticky situations with ease. Zhonya's Hourglass
Buy this if the enemy team is heavy on AD. The active is also really helpful for surviving that couple of seconds that is on cooldown if you get stuck. It can also help avoid certain skills like Lux's Finales Funkeln or Karthus's Requiem Archangel's Staff
If the game goes on even after you have your second Archangel's or if you just aren't keeping stacks up, sell your Soulstealer for a third AA. Your AP will be insane, and when this is combined with the spell pen provided by my build you'll do absurd amounts of damage. Banshee's Veil/ Rod of Ages
If you're up against a difficult champion in mid or just want more sustain in the lane, buy a Catalyst the Protector and build it into either of these items later. Both are great on Kassadin; I just prefer more damage to survivability and lane sustain. Abyssal Mask
If the enemy team is stacking MR, the spell pen aura can be helpful for you and the rest of your team. Even if they aren't, the magic resist could help against an AP-heavy team.

*UPDATE* Dominion Build

The build is a fairly standard AP build abridged for Dominion. Nothing out of the ordinary here.
Start with Boots of Speed and your Tear of the Goddess. Hit the speed boost and then top. their casters as constantly as you can and pick any weakened targets if you can. If your team cannot take top and starts losing the fight, retreat before the other team's DPS kills you.
Throughout the game, use your ALL THE TIME. You can charge up multiple stacks on your way to save a point that's being captured and drop someone instantly if you have your timing right. You can escape over walls (sooooo many walls).

*Update* Dominion Build 2

A bit of an unusual/experimental playstyle. This one is inspired by Azrakal's comment. He suggested a Morello's Evil Tome for my Summoner's Rift build. I thought about it, and I decided that the active on Deathfire Grasp is worth the slightly lessened stats. When I buy DFG, I move it to item slot 1. Kassadin's burst rotation then becomes: in, , , , , autoattack. To chase: on top of them and autoattack, hitting W whenever it's up. This rotation can often one-shot squishy enemies.

Final Words

Thanks for taking the time to read my guide. Leave me a vote or a comment to tell me what you think, but don't rag on me too hard. This is my first guide, after all. Good luck, have fun, and I'll see you on the Fields of Justice!